Fixes#11705
Add bg_color and fg_color options to GhosttyRenderStateRowCellsData that
resolve the final RGB color for a cell, flattening the multiple possible
sources. For background, this handles content-tag bg_color_rgb,
content-tag bg_color_palette (looked up in the palette), and the style
bg_color. For foreground, this resolves palette indices through the
palette; bold color handling is not applied and is left to the caller.
Both return GHOSTTY_INVALID_VALUE when no explicit color is set, in
which case the caller should fall back to whatever default color it
wants (e.g. the terminal background/foreground).
ecc55b94c8